The method herein relates to reducing the total nitrogen content of shale oil by reducing the viscosity of the shale oil and then extracting nitrogen-containing compounds from the shale oil with an organic acid.
More particularly, this application relates to a method for reducing the nitrogen content of shale oil produced in either an above-ground or in an in situ oil shale retort.
The term "oil shale" as used in the industry is, in fact, a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising marlstone deposits with layers containing an organic polymer called "kerogen" which, upon heating, decomposes to produce liquid and gaseous products. The formation containing kerogen is called "oil shale" herein and the liquid product produced upon decomposition of kerogen is called "shale oil".
Kerogen is considered to have been formed by the deposition of plant and animal remains in marine and nonmarine environments. Its formation is unique in nature. Alteration of this deposited material during subsequent geological periods produced a wide variety of organic materials. Source material and conditions of deposition were major factors influencing the type of final product formed.
Kerogen samples, found in various parts of the world, have nearly the same elemental composition. However, kerogen can consist of many different compounds having differing chemical structures. Some compounds found in kerogen have the structures of proteins while some have structures of terpenoids, and others have structures of asphalts and bitumens.
Shale oils produced from oil shale are generally high molecular weight, viscous organic liquids, of predominantly hydrocarbonaceous oxygen, nitrogen and sulfur or containing organic compounds. The shale oils are of varying linear, branched cyclic aromatic hydrocarbon and substituted hydrocarbon content with high pour points, moderate sulfur content and relatively high nitrogen content. As the composition of shale oil depends upon the composition of the kerogen within the oil shale formation, the composition of the shale oil can vary from one geographic location to another. The shale oil produced from an oil shale formation can vary also between strata within the oil shale formation. The nitrogen content of shale oil can also vary dependent upon the geographical location of the oil shale deposit from which the shale oil is produced. Such a variance in nitrogen content in different geographical locations can be attributed to differences in the environment during the time of the deposition of the organisms which, upon lithification, became oil shale. Such a variance can also be attributed to the different types of organisms in the separate geographical locations which were deposited to form the organic substance in the oil shale and any organisms within the formed deposit layer which acted upon such deposited material to provide the kerogen within the oil shale formation.
The nitrogen content in shale oil is attributable to basic nitrogen-containing compounds and nonbasic nitrogen-containing compounds. The relative percentages of the basic and non-basic nitrogen compounds comprising the total nitrogen content of a shale oil can also vary depending upon the particular shale oil.
The nitrogen content of shale oil is generally up to about two percent by weight. The average nitrogen content of shale oil recovered by in situ retorting of oil shale from the Piceance Creek Basin of Western Colorado is on the order of about 1.4 percent by weight.
The presence of nitrogen in shale oil presents many problems in that the nitrogen can interfere with the transportation and use of the shale oil. Deleterious effects brought about by the presence of nitrogen in shale oil are decreased catalyst life in dehydrogenation, reforming, hydrocracking and catalytic cracking reactions, decreased chemical stability of products, and decreased color stability of products. Another problem with the presence of nitrogen in shale oil is that it is undesirable to transport nitrogen-containing shale oil through pipelines which are also used for transporting petroleum products because of possible pollution of such products with residual nitrogen-containing shale oil in the pipeline. Generally such petroleum products contain a very low nitrogen content. The relatively high nitrogen content in the shale oil can pollute the pipelines making them undesirable and uneconomical for transporting such low nitrogen-containing petroleum products. In addition, high nitrogen content in shale oil can cause clogging of pipelines due to self-polymerization brought about by the reactivity of the nitrogen-containing compounds. Due to the basicity of the nitrogen-containing compounds in shale oil some corrosion can occur thus damaging a pipeline used to transport shale oil.
Product stability is a problem that is common to many products derived from shale oil with the major exception of the asphalt cut and those products that have undergone extensive hydrotreating. Such instability, including photosensitivity, is believed to be resultant, primarily from the presence of nitrogen-containing compounds.
It is, therefore, desirable to reduce the nitrogen content of shale oil to increase the utility, transportability, and stability of the shale oil and the products derived from such shale oil.
Due to the undesirable nature of nitrogen in organic fluid streams, such as fluid streams produced in the recovery and refining of petroleum, coal and oil shale, many processes have been developed to reduce the nitrogen content to an acceptable level. The level of acceptability for the nitrogen content is generally based upon the use of the particular stream.
In U.S. Pat. No. 3,719,587 to Karchmer et al a process is disclosed for removing basic nitrogen-containing compounds from coal naphtha. The basic nitrogen compounds are removed by washing the naphtha with water or with a dilute aqueous solution of a strong acid. The dilute acid solutions are disclosed as sulfuric acid, hydrochloric acid, phosphoric acid and acetic acid.
U.S. Pat. 2,848,375 to Gatsis discloses a process for removing basic nitrogen compounds from organic substances by washing with a weak acid in combination with a polyalcohol. The weak acid used is boric acid in combination with a polyhydroxy organic compound which has hydroxyl groups on adjacent carbons.
U.S. Pat. No. 2,741,578 to McKinnis teaches that mineral oils can be treated to recover the nitrogen bases by extracting the mineral oils with a selective solvent for the nitrogen bases. The selective solvents are organic hydroxy compounds. Organic hydroxy compounds which can be used are the compounds which have a pH greater than 6.5.
U.S. Pat. No. 2,035,583 to Bailey discloses a process for the separation and recovery of nitrogen bases from mineral oils. In the process, the mineral oil is extracted with a solvent for the nitrogen bases. Acceptable solvents are liquid sulfur dioxide, furfural, aniline, nitrobenzene and isobutyl alcohol. However, due to the solubility of desirable mineral oils, such as aromatics and olefines, the process also includes extracting the resultant extract with dulite aqueous acids to recover the nitrogen bases from the first extract. The nitrogen bases are then recovered from the aqueous solution by adding an inorganic base to precipitate the nitrogen bases.
U.S. Pat. No. 2,035,012 to Stratford et al discloses a process for improving the color and viscosity of petroleum oils. In the process an oil is extracted with a selective solvent in combination with an acid. The selective solvent can be phenol, nitrobenzene, furfural or liquid sulfur dioxide. The acid is preferably an inorganic acid but can also be an organic acid such as picric, acetic, oxalic, citric and benzene sulfuric acids.
U.S. Pat. No. 2,541,458 to Berg discloses a process for recovery of nitrogen bases from hydrocarbon fractions. In the process the fraction is extracted with a volatile acid or nonvolatile acid salt in combination with a mutual solvent for the acid and the hydrocarbon fraction. The mutual solvents include low boiling alcohols and ketones. The extraction is conducted in the presence of water to avoid loss of the volatile acids.
U.S. Pat. No. 2,309,324 to McAllister et al discloses a method for removing nitrogen bases from water-insoluble organic solvents, mineral oils and hydrocarbon fractions. In the process the mineral oil is extracted with an aqueous, weak acid solution. The weak acids are classified as acids having dissociation constants below 10.sup.-3. The aqueous acid solutions are prepared by dissolving from 15 to 90 weight percent of an acid in water. Upon extraction of the oil, two phases are formed. The aqueous phase contains the acid and absorbed nitrogen bases. The other phase consists of the organic substance from which at least a portion of the nitrogen bases has been removed.
Many of the processes described in the above patents do not address themselves to the removal of nonbasic nitrogen compounds which can be present in organic fluids. Additionally, many of the above described processes are not specific for treatment of shale oil and the relatively high nitrogen content found in shale oil. Still further, none of the above processes are specific for lowering the nitrogen content for shale oil produced by in situ retorting of oil shale.